How can I fix the knob on my steam cleaner, reliable, enviromate 3?

My knob to turns steam cleaner on and off which is on the tank (or base) of machine has popped up. If I hold the knob down the machine light comes on and machine will heat up but if I take my finger off the knob it pops up again and machine turns off. Please help this steamer was 500 bucks and I'm put of warranty The company wants 200 bucks just to ship it back and forth and diagnose problem Plus additional costs to fix knob